I've placed a building near Falkreath and I like where it's placed. I started landscape editing so I could remove the grass and there's a chunk of a cell, about half, that refuses to be painted. I'll paint in adjacent cells and it will change. It's so bad that you can see a straight, unbroken line between the two paints. I have no idea what's going on. I really don't want to resort to lowering the landscape and building on top of it to hide the grass. Any help is appreciated.

Probably you have more than 7 textures active on that quadrant of a cell. Look up the tutorial on TES Alliance that goes into detail onhow to handle this.
http://tesalliance.org/forums/index.php?/topic/4663-ck-basics-lesson-3/

You're a life saver.